Heavy-duty gas turbines with can-annular combustion systems require a liquid fuel delivery system which can control total flow rate to and divide fuel flow accurately and equally among multiple combustion chambers. A can-annular combustion system consists of an arrangement of combustors about the turbine axis, each with a fuel injection system and a transition piece which provides a flow path for the hot gas from the combustor to the inlet of the turbine. Unequal fuel flow distribution to the combustion chambers can lead to temperature non-uniformity about the turbine which in turn can reduce turbine life.
Heavy-duty gas turbine fuel delivery systems with can-annular combustors conventionally include, as major components, a positive displacement fuel pump, a bypass flow control valve, a mechanical flow divider and a controller. The positive displacement pump is driven by the turbine's accessory gear box or by an electric motor. The bypass control valve is positioned by the turbine controller to maintain the desired total liquid fuel flow rate for delivery to the flow divider. The mechanical flow divider, in turn, uses a synchronized arrangement of paired gears to discharge the liquid fuel to the various combustors. Each pair of gears discharges fuel to a single combustor. The gears are identical and rotate at the same speed. Hence, the volumetric flow rate discharged to each combustor is nominally equal. A rotational speed feedback device, i.e., magnetic pick-ups which sense rotation, is installed in the flow divider and provides a signal to the turbine controller proportional to the fuel flow rate. The controller uses the flow divider feedback signal in control algorithms to correctly position the bypass control valve, as well as to carry out other control functions.
The flow divider also contains components which are in constant high speed rotary motion and are heavily loaded. Gears and bearings within the mechanical flow divider are lubricated by the liquid fuel itself. When low lubricity fuels are used, such fuels must be dosed with a fuel additive in order to avoid failure of the flow divider due to premature wear, galling, etc. However, these lubricity additives build up on the surfaces of the flow divider, causing it to bind up after a modest period of operation. The flow divider must then be removed from service, cleaned and overhauled. The additive also builds up on the internal surfaces of check valves downstream of the flow divider, causing them to leak or bind. The additive itself is costly and not always readily available in remote regions of the world.
Existing systems use a positive displacement fuel pump specifically designed for operation with low lubricity fuels. There are drawbacks when using a positive displacement type fuel pump. Pumps of this type have long manufacturing lead times, are expensive as compared to standard distillate fuel pumps, require special bearing lubricating oil system and have less than desired reliability. The pump is a special design with few other applications, and spare parts are not typically readily available. However, there are a very substantial number of low lubricity fuel systems using positive displacement pumps of this type and is not always particularly cost-effective to replace such positive displacement type pumps in existing systems. In new systems employing original equipment, however, other types of pumps can be economically substituted with positive advantages.
The problem addressed herein is to configure the fuel delivery system so as to control and divide fuel flow while accommodating a wide range of liquid fuel types, including fuels with poor lubricating properties and which system offers high reliability, low purchase and operating costs and low maintenance requirements. It is thus desirable to retrofit existing systems without replacement of the high pressure fuel pump, and also to offer original equipment systems which likewise eliminate the lubricity dosing problem and substitute a high reliability conventional centrifugal pump for the specially designed high pressure positive displacement fuel pump.